A planar electromagnetic detector wherein a semiconductor film and a semiconductor sensor are arranged in a planar array, and switching elements are disposed at respective pixel electrodes is known conventionally. The semiconductor film is provided for generating charges upon detecting x-ray or other electromagnetic wave. The semiconductor sensor is mainly composed of a semiconductor film having electromagnetic conductivity and pixel electrodes (charge collection electrodes). In this conventional electromagnetic detector, the switching elements are activated line by line so as to read out the charges as generated line by line.
The detailed structure and principle of a planar image detector corresponding to the foregoing electromagnetic detector are disclosed, for example, in “D. L. Lee, et al., “A New Digital Detector for Projection Radiography”, SPIE, 2432, pp. 237-249, 1995. The principle of this planar image detector will be explained in reference to FIG. 17.
As illustrated in FIG. 17, the planar image detector includes a semiconductor film 201 made of Selenium (Se) having electromagnetic conductivity, a common bias electrode 202 of a single layer formed on the semiconductor film 201, and a plurality of charge collection electrodes 203 formed under the semiconductor film 201. Each of the plurality of charge collection electrodes 203 is connected to a charge storage capacitor (Cs) 204 and a TFT element (active element) 205. Further, dielectric layers 206 and 207 which serve as electron blocking layers are formed between the semiconductor film 201 and the bias electrode 202, and between the semiconductor film 201 and the charge collection electrodes 203 respectively. To the bias electrode 202, a high voltage power supply 209 is connected. In FIG. 17, a reference numeral 208 indicates an insulating substrate.
In the above planar image detector, the semiconductor film 201 generates therein charges (electron-hole pairs) in response to x-ray or other electromagnetic wave incident thereon. In this state, the semiconductor film 201 and the charge storage capacitors 204 are connected in series. Therefore, with an applied bias voltage to the bias electrode 202, charges (electron-hole paris) as generated from the semiconductor film 201 are moved to the positive electrode side and the negative electrode side respectively, whereby charges are stored in the charge storage capacitors 204.
The charges as stored in the charge storage capacitors 204 can be read into an external equipment by activating the TFT elements 205. As described, in the foregoing planar image detector, the charge collection electrodes 203, the charge storage capacitors 204 and the TFT elements 205 are arranged in a planar array, and two-dimensional information of electromagnetic wave to be detected can be obtained by reading out the charges line by line in order.
For the semiconductor film 201 having electromagnetic conductivity, Se, CdTe, CdZnTe, PbI2, HgI2, SiGe, Si or similar material is typically adopted. Among all, Se (an amorphous a-Se film in particular) offers low dark current (leak current), and permits a formation of a laminated film of a large area by vacuum deposition at low temperature. For this beneficial characteristic, an Se film is often adopted for the semiconductor film 201 of electromagnetic detectors (x-ray detectors in particular) of the structure wherein the semiconductor film 201 is formed directly over the active matrix substrate 210 (see FIG. 17).
FIG. 18 is a longitudinal sectional view illustrating the detailed structure around one pixel in the foregoing electromagnetic detector.
As illustrated in FIG. 18, an active matrix substrate 210 of the electromagnetic detector includes the insulating substrate 208 made of, for example, a glass substrate, having formed thereon a gate electrode 211, a charge storage capacitor (Cs) electrode 212, a gate insulating film 213, a connection electrode (drain electrode) 214, a data electrode (source electrode) 215, an insulating protective film 216, a TFT element 205, an interlayer insulating film 217, and a charge collection electrode (pixel electrode) 203. In this interlayer insulating film 217, formed is a contact hole 218 for connecting the charge collection electrode (pixel electrode) 203 to the connection electrode 214. The electromagnetic detector further includes the semiconductor film 201 and the bias electrode 202 formed over the active matrix substrate 210.
In the active matrix substrate 210 used in the electromagnetic detector, generally, a glass substrate is adopted as the insulating substrate 208 whereon electric lines and the TFT elements 205 or other essential members are prepared by laminating a metal film (Al, Ta, etc.), a semiconductor film (a-Si or p-Si, etc.), an insulating film (SiNx, SiO, etc.), and further carrying out the patterning of the resulting multi-layer structure in shapes as desired.
However, for the electromagnetic detector wherein an a-Se film as the semiconductor film 201 is formed over the active matrix substrate 210 adopting a glass substrate as the insulating substrate 208, there exists approximately 1-digit difference in thermal expansion coefficient between the thermal expansion coefficient 3 to 8 (×10−6/° C.) of the glass substrate and the thermal expansion coefficient 30 to 50 (×10−6/° C.) of the a-Se film, leading to the problem that the peeling of the a-Se film is liable to occur with changes in temperature.
The peeling of the a-Se film is liable to occur also by an external load exerted, for example, in a direction of bending the insulating substrate 208 Particularly, such peeling of the a-Se film is more liable to occur from the outer circumference of the a-Se film. Therefore, when adopting a large-size screen for the foregoing electromagnetic detector, the problem of the peeling of the a-Se film becomes more outstanding due to a larger difference in thermal expansion coefficient between the a-Se film and the insulating substrate 208, and the resulting warpage of the insulating substrate 208.
The above-explained peeling of the a-Se film if occurs leads to the following problem. That is, charges as generated from the a-Se film in response to, for example, x-ray incident thereon cannot reach the charge collection electrode 203 of the TFT element 205, and thus the detection of x-ray becomes impossible. Furthermore, in an even that the a-Se film starts being peeled from the outer circumference, the a-Se film is liable to be peeled even from the pixel array region, leading to serious damages.